Solar Water in the Desert

Scientists have developed a promising technology to bring much-needed fresh water to arid regions.
8 August, 2017
Climate change has only heightened the awareness of a challenge that researchers around the world have long contemplated: how do you wring water from the air in places with no other options?
They've met with some success in places like Chile or Morocco, where fog harvesting has been used, but that – according to MIT scientist Evelyn Wang – requires a relative humidity of 100 percent, and those conditions rarely exist in the desert. Another limited success comes with dew harvesting, which mimics what most people see when they enjoy a cold beverage in a glass container on a hot summer day.

Capturing that water is possible but prohibitively expensive, so it's not really a solution for the needs of millions of people, many who live in the developing world. A promising new technology may be the key, and aluminium plays a starring role in making future models of the "water maker" far less expensive.
The MIT and University of Berkeley scientists on Wang's team have built what is basically a high-tech, solar powered sponge. It alternatively can work with a biomass energy source, because all it needs is some source of heat to support the chemical process caused by metal-organic frameworks (MOFs).

So far, the team's device yields 2.8 liters of water per day for every kilogram of the MOF absorbent used. It's entirely passive, so people can just set it up at night and then during the day the box makes water. It works in extremely dry ranges and has performed at as low as 20 percent humidity. That's the daily July average at Death Valley in the American Southwest, comparable to the Kalahari or seasonal Sahara.
Image: Science
MOFs are not new. They were discovered by Berkeley chemistry professor Omar Yaghi some two decades ago, and belong to a family of engineered compounds designed like sponges. That means they have very large 3D surface areas for absorption, and with targeted fine-tuning of the metals in the mix, an MOF can be specifically built to attract water. There have been more than 20,000 MOFs invented for different reasons, but in 2014 Yaghi discovered an MOF seen as a solution to the world's water woes.

When an MOF "sponge" made with zirconium is placed between a top surface painted black to absorb solar heat, and a lower panel kept at air temperature, water vapor drips down and is collected below it. During the day, users collect water a few times and open the device to let in fresh air and start it anew. It's what Yaghi calls "personalized water" using a method that is entirely off-grid and ideally ecofriendly.
Image: Science
At any given time, the amount of water that's on the planet – but in the air – is equal to about 10 percent of the known freshwater sources in lakes and rivers. Tapping into it is a climate game-changer.

Now that the team has proved that the science works, they're moving on to the next stage. The problem with zirconium is that it's so expensive. It costs about USD$150 a kilogram, and that makes it prohibitive to deploy water harvester boxes widely – and in the places where they're needed most. The good news, Yaghi says, is that his team has already had early success in building MOFs for water absorption that rely on aluminium instead. It's about 100 times cheaper, and would make the "water box" easy to scale up.
Banner image: New York Post